High-efficiency dissipative Kerr solitons in microresonators

The microresonator comb (microcomb) is a laser source that generates equally spaced coherent lines in the spectral domain. Having a chip-scale size and the potential of being low cost, it has attracted attention in multiple applications. Demonstrations have included high-speed optical communications, light detection and ranging, calibrating spectrographs for exoplanet detection and, optical clocks. These experiments typically rely on the generation of a dissipative Kerr soliton (DKS) --- a temporal waveform that circulates the microresonator without changing shape. However, these DKS states have thus far been limited in certain technical aspects, such as energy efficiency, which are essential for realizing commercial microcomb solutions.This thesis studies the dynamics of DKSs in microresonators aiming at developing a reliable and high-performing microcomb source. The investigation will cover DKSs found both in the normal and anomalous dispersion regime of silicon nitride microresonators. The performance of microcombs in terms of line power is numerically explored in single-cavity arrangements for telecommunication purposes. DKSs generated in linearly coupled microcavities are investigated, revealing exotic dynamics and improved performance in terms of power efficiency and DKS initiation. These studies facilitate reliable energy-efficient microcombs, bringing the technology a step closer to commercial use

Bidirectional initiation of dissipative solitons in photonic molecules

Dissipative solitons (DSs) can be generated in microresonators featuring Kerr nonlinearities via continuous wave (CW) pumping, forming a frequency comb in the spectral domain. While single cavity DSs have been thoroughly investigated in the last years, recent efforts have moved towards photonic molecules (linearly coupled cavities). These arrangements give rise to exotic physical phenomena and practical improvements in terms of conversion efficiency and tuneable comb dynamics. In a recent study of normal dispersion photonic molecules, we found that DSs can be generated in absence of intracavity CW bistability. Here, we show that this feature enables the CW initiation of DSs, tuning the laser into resonance either from the blue side or the red side. While DS initation from the red side has been demonstrated with the photorefractive effect, this is the first demonstration of bidirectional initiation that only requires a Kerr nonlinear medium

Widely Tunable and Narrow Linewidth Laser Source based on Normal-Dispersion Frequency Combs and Optical Injection Locking

By injection-locking tones of a normal-dispersion, photonic molecule enabled microcomb, a tunable laser source is demonstrated with > 55 nm of tunable range, < 8 kHz integrated linewidth, > 5 dBm of power, and > 60 dB SMSR

Widely tunable narrow linewidth laser source based on photonic molecule microcombs and optical injection locking

We demonstrate a method to generate a widely and arbitrarily tunable laser source with very narrow linewidth. By seeding a coupled-cavity microcomb with a highly coherent single-frequency laser and using injection locking of a Fabry-Perot laser to select a single output comb tone, a high power, high side mode suppression ratio output wave is obtained. The system is demonstrated across 1530 -1585 nm with a linewidth below 8 kHz, having 5 dBm output power and sidemode suppression of at least 60 dB. Prospects of extending the performance are also discussed

Frequency-Comb-Assisted Swept-Wavelength Interferometry

Swept-wavelength interferometry (SWI) is a highly sensitive and versatile technique implemented in a diverse array of industrial and scientific applications. SWI uses a continuously tunable laser to capture the magnitude and phase response of a device under test (DUT). The prevalent non-linear tuning of the laser calls for an auxiliary interferometer for the calibration of the laser frequency on the fly [1]. However, this approach is susceptible to environmental perturbations, and the inherent dispersion of the interferometer introduces systematic errors. Laser frequency combs can be used as optical rulers against which to calibrate tunable lasers with high- precision and, when self-referenced, with high accuracy [2]. Here, we apply this comb-based calibration approach in the context of SWI for the first time and illustrate its relevance for the characterization of high-Q microresonators

Low-loss high-Q silicon-rich silicon nitride microresonators for Kerr nonlinear optics

\ua9 2019 Optical Society of America. Silicon nitride is a dielectric material widely used for applications in linear and nonlinear optics. It has an ultra-broad transparency window, low intrinsic loss, and a refractive index that allows for moderate optical field confinement in waveguides. The chemical composition of this material can be precisely set during the fabrication process, leading to an extra degree of freedom for tailoring the optical and mechanical properties of photonic chips. Silicon-rich silicon nitride waveguides are appealing for nonlinear optics, because they have a higher nonlinear Kerr coefficient and refractive index than what is possible with stoichiometric silicon nitride. This is a direct consequence of the increased silicon content. However, silicon-rich silicon nitride waveguides typically display higher absorption losses. In this Letter, we report low-loss (∼0.4 dB∕cm) silicon-rich silicon nitride waveguides. The structures feature high optical confinement and can be engineered with low anomalous dispersion. We find an optimum silicon composition that, through an annealing process, overcomes optical losses associated to N-H bonds in the telecom band. Based on this technology, we successfully fabricate microresonators with mean quality factors (Q) ∼0.8 7 106 in the C and L bands. Broadband coherent microresonator frequency combs are generated in this platform, indicating its potential for efficient Kerr nonlinear optics

Low Loss Silicon-Rich Silicon Nitride for Nonlinear Optics

We demonstrate low loss (~ 0.4 dB/cm) silicon-rich silicon nitride waveguides and highQ microresonators (Qi ~ 1 000 000) featuring broadband anomalous dispersion. Microresonator combs aregenerated for the first time in this emerging material platform

Widely Tunable and Narrow Linewidth Laser Source based on Normal-Dispersion Frequency Combs and Optical Injection Locking

By injection-locking tones of a normal-dispersion, photonic molecule enabled microcomb, a tunable laser source is demonstrated with > 55 nm of tunable range, < 8 kHz integrated linewidth, > 5 dBm of power, and > 60 dB SMSR

Surpassing the nonlinear conversion efficiency of soliton microcombs

Laser frequency combs are enabling some of the most exciting scientific endeavours in the twenty-first century, ranging from the development of optical clocks to the calibration of the astronomical spectrographs used for discovering Earth-like exoplanets. Dissipative Kerr solitons generated in microresonators currently offer the prospect of attaining frequency combs in miniaturized systems by capitalizing on advances in photonic integration. Most of the applications based on soliton microcombs rely on tuning a continuous-wave laser into a longitudinal mode of a microresonator engineered to display anomalous dispersion. In this configuration, however, nonlinear physics precludes one from attaining dissipative Kerr solitons with high power conversion efficiency, with typical comb powers amounting to ~1% of the available laser power. Here we demonstrate that this fundamental limitation can be overcome by inducing a controllable frequency shift to a selected cavity resonance. Experimentally, we realize this shift using two linearly coupled anomalous-dispersion microresonators, resulting in a coherent dissipative Kerr soliton with a conversion efficiency exceeding 50% and excellent line spacing stability. We describe the soliton dynamics in this configuration and find vastly modified characteristics. By optimizing the microcomb power available on-chip, these results facilitate the practical implementation of a scalable integrated photonic architecture for energy-efficient applications

Multilayer integration in silicon nitride: decoupling linear and nonlinear functionalities for ultralow loss photonic integrated systems

Silicon nitride is an excellent material platform for its extremely low loss in a large wavelength range, which makes it ideal for the linear processing of optical signals on a chip. Moreover, the Kerr nonlinearity and the lack of two-photon absorption in the near infrared enable efficient nonlinear optics, e.g., frequency comb generation. However, linear and nonlinear operations require distinct engineering of the waveguide core geometry, resulting in a tradeoff between optical loss and single-mode behavior, which hinders the development of high-performance, ultralow-loss linear processing blocks on a single layer. Here, we demonstrate a dual-layer photonic integration approach with two silicon-nitride platforms exhibiting ultralow optical losses, i.e., a few dB/m, and individually optimized to perform either nonlinear or linear processing tasks. We demonstrate the functionality of this approach by integrating a power-efficient microcomb with an arrayed waveguide grating demultiplexer to filter a few frequency comb lines in the same monolithically integrated chip. This approach can significantly improve the integration of linear and nonlinear optical elements on a chip and opens the way to the development of fully integrated processing of Kerr nonlinear sources